The present invention relates to a snap fastener with a differentiated buttoning and unbuttoning effort female part.
The field of the invention is that of the automatic snap fasteners, in which the stem of the snap fastener male part is engaged in the snap fastener female part and is held therein by a holding spring.
In such a device, it is necessary to provide a given resistance of snap fastener against its “opening” or unbuttoning, without however hindering the buttoning up operation which, on the contrary, should be as fast and easy as possible.
Commercially available snap fasteners do not provide any difference between the buttoning up and unbuttoning effort, thereby the effort required to open or unbutton the snap fastener is normally about the same as that necessary for closing or buttoning it.
The above prior snap fasteners, however, have the drawback that, since the snap fastener should provide a firm tightness after its closure, the buttoning effort is a comparatively high one, thereby rendering the snap fastener closing or buttoning operation a rather difficult one, mainly at positions without a biasing abutment or with a yielding abutment.
Document US 2014/109352 A1 discloses a snap fastener, wherein the opposite shoulders of a chamber including the snap fastener female part spring have a substantially like length, thereby the engaging of snap fastener stem in the female part causes an exclusively radially extending deformation of the spring.
Document EP 2 441 340 A1 discloses a snap fastener including a spring housed in a chamber, which chamber has, in turn, a shoulder on a buttoning side which is shorter than a corresponding shoulder on the opposite side. Since this prior snap fastener requires a comparatively high closing or buttoning effort, said spring is formed with built-in resilient deformable tongues so designed as to facilitate the snap fastener closing operation, while having however the disadvantage of reducing the snap fastener tightness or effort against an accidental and undesired opening thereof.